In WDM transmission systems, a plurality of channels are conveyed simultaneously over the same fiber, each of the channels being carried by an associated wave having a determined center wavelength and a determined bandwidth. In particular, for performing routing or switching functions, it is often necessary to drop certain channels and to modify the multiplex, e.g. so as to modify the information conveyed by one or more channels, or so as to replace the wavelength carrying the information with some other wavelength.
As shown diagrammatically in FIG. 1, in order to implement a drop-and-insert multiplexer, it is possible to use a stop filter 1 associated with drop coupling means C1 and with insert coupling means C2. The filter is organized to prevent transmission of at least one of the wavelengths .lambda.k of the multiplex, and also to reflect it/them towards the drop coupling means C1. The drop-and-insert multiplexer becomes reconfigurable if the filter is itself reconfigurable as a function of a channel selection command CD.
In the example shown, the multiplexer is essentially constituted by the reconfigurable filter 1 provided with two opposite access ports Pa, Pb. The ports Pa and Pb are coupled respectively to the inlet coupling means C1 and to the outlet coupling means C2. In this example, the coupling means are three-port circulators. The inlet circulator C1 is provided with an inlet port P12 suitable for receiving an input multiplex We. A second port P2 is coupled to the first access port Pa of the filter 1. A third port P3 constitutes a drop port. The circulator C1 is disposed in a manner such as to enable the input multiplex We to be fed into the filter 1 via the first access port Pa, while the wave Wd reflected by the filter 1 and exiting via the same port Pa is received by the port P2 so as to be taken via the drop port P3.
In the same way, the outlet circulator C2 is provided with a first port P5 coupled to the filter 1 via the second access port Pb. A second port P4 of the circulator C2 makes it possible to take the wave exiting from the filter via the port Pb. The port P4 constitutes an outlet port making it possible to take an output multiplex Ws. Finally, the circulator C2 is provided with a third port P6 which constitutes an insert port.
As a function of the command CD that is applied, the filter 1 is configured to have a stop comb or band set to one or more determined wavelengths .lambda.k of the multiplex.
In operation, an input multiplex We is fed in via the inlet port P1. The channels Wd of the multiplex that are carried by the wavelengths stopped by the filter are than dropped and accessible via the drop port P3. In addition, by injecting new signals Wa carried by the same wavelengths via the insert port P6, corresponding channels are inserted into the output multiplex Ws.
Conventional solutions for making a reconfigurable filter use fixed elementary filters associated with 2-to-2 optical switches of the"crossbar" type.
FIG. 2 shows a first conceivable example of such a solution.
The filter includes n 2-to-2 switches X1, X2, Xk, Xn connected in cascade so that one of the outlets of each switch is connected to one of the inlets of the next switch. In addition, the second inlet of each switch is connected to its second outlet via an associated stop filter F1, F2, Fk, Fn. Generally, each of the filters is organized to reflect a single one of the wavelengths .lambda.1, .lambda.k, .lambda.n of the inlet multiplex We, but for certain applications, it is also possible to use stop filters that stop combs of wavelengths corresponding respectively to determined subsets of channels.
For example, the 2-to-2 optical switches include moving optical waveguides, such as fiber segments, that can be actuated electro-mechanically, e.g. by means of electric micromotors or piezoelectric elements. The coupling between switches uses optical fibers and the stop filters can be made in the form of Bragg gratings photo-inscribed in the coupling fibers.
FIG. 3 shows another example based on a 1-to-2 optical switch X1, 2-to-2 optical switches Xk, Xn, and a 2-to-1 optical switch SW. In this example, the second outlet of each of the n first switches is connected to the second inlet of the next switch via a stop filter.
As explained in detail below, those solutions suffer from the drawback of not being optimal with respect to the insertion losses generated by the switches.